FIG. 1 shows a side view of an example of a conventional gas turbine 10 used in power generation. The gas turbine 10 includes a compressor 20, combustion chamber 30, turbine 40, and exhaust diffuser 50. In operation, air enters air intake 5, where it is compressed by the compressor 20 and directed into the combustion chamber 30. In the combustion chamber 30, the compressed air is mixed with fuel gas and injected into the combustion chamber 30 via nozzles 25. The mixture of compressed air and gas is burned within the combustion chamber 30 to generate a high-temperature, high-pressure combustion gas, which is used to drive the blades of the turbine 40 to thereby cause rotation of the rotor, to which the blades are attached. Exhaust gas from the turbine 40 is directed into an exhaust diffuser 50.
FIG. 2 shows a cross-sectional view of an example of an exhaust cylinder section 50 connected to a discharge of a turbine cylinder section 40 to receive therefrom exhaust gas 79 from the last row of turbine blades 55. The exhaust cylinder section 50 comprises an exhaust cylinder or outer diffuser shell 52 and an inner diffuser shell 61, which enclose a generally cylindrical inner shell 72. From the exhaust section 50, the hot gas 79 output by the turbine cylinder section 40 may be directed, for example, to a heat recovery steam generator (HRSG).
The journal bearing housing 72 and inner diffuser shell 61 form a portion of the flow path 81 for the hot gas 79. The journal bearing housing 72 supports a rotor 56 via bearing 60. Struts 57 support the bearing housing and extend between the bearing housing and the outer diffuser shell 52 and which connect to the outer diffuser shell 52 at their distal ends 71. The portion of the struts 57 between the inner shell 72 and the exhaust cylinder or outer diffuser shell 52 are enclosed by a shield 58 to protect the struts 57 from the exhaust gas.
As shown in FIG. 2, the exhaust cylinder section 50 also includes an exhaust manifold outer cylinder 53 extending downstream from the exhaust cylinder 52. The exhaust manifold outer cylinder 53 is bolted at its upstream flange 63 to the exhaust cylinder 52 downstream flange 64. A flow guide 62 extends from the exhaust manifold outer cylinder 53 inboard of the flange 63 to smooth a boundary flow path of exhaust gas from the inner diffuser shell 61. Likewise, an exhaust manifold inner cylinder 54 extends downstream from the inner cylinder 72. The turbine cylinder 51 is bolted to the upstream flange 80 of the exhaust cylinder 52. A shroud 69 attached to the turbine cylinder encircles the tips of the last row of turbine blades 55.
FIG. 2 shows an annular cavity 67 formed between the inner diffuser shell 61 and the exhaust cylinder or outer diffuser shell 52, the inner diffuser shell 61 forming a boundary between the hot gas path 81 and the annular cavity 67. A seal 65 is provided to extend between the OD of the inner diffuser shell 61 rear flange 66 and the ID of the exhaust cylinder 52 rear flange 64 and to extend a full 360 degrees around the cavity 67 to obstruct flow through the cavity.
As can be readily appreciated from the construction noted above, access to, for example, the seal 65 requires dismantling of the exhaust manifold outer cylinder 53 from the exhaust cylinder 52 and corresponding movement of such sections, together with associated movement of the exhaust cylinder 52 and/or inner diffuser shell 61.
For years certain industrial gas turbines (e.g., the 501F gas turbine, the 501G gas turbine) have encountered recurring failures of components such as the air baffles and dead air space baffles. Despite numerous iterations of attempts to fix this problem by the manufacturer of the gas turbine and by the gas turbine industry users group (the air baffles are on the 6th design iteration and the dead air space baffles are on the 2nd design iteration), both the original design and the subsequent design iterations have all proven susceptible to failure, often with only about 6 months of service.
FIG. 4A shows a perspective view of the exhaust cylinder section of a 501F gas turbine showing the exhaust cylinder 100, the diffuser shell 110, the bearing support 115, struts 118, strut shielding 119, and dead air space baffles 120.
FIG. 5 shows a close-up perspective view of a portion of FIG. 4A, showing in more detail portions of the exhaust cylinder 100, diffuser shell 110, and dead air space baffles 120. FIG. 5 shows that the dead air space baffles 120 are connected, at a distal end, to the flange 102 of the exhaust cylinder 100 via slot 101 and are connected, at a proximal end, to diffuser shell 110 via the holding ring 130. The holding ring 130 is, in turn, held in place using a plurality of bolts extending through bolt holes 131, 121 and 111, formed in the holding ring, dead air space baffle, and diffuser shell, respectively. The dead air space baffles 120 prevent hot exhaust gas from flowing over the bearing support struts in the annular cavity formed between the inner diffuser shell and the exhaust cylinder (see, e.g., FIGS. 4A-4B). In this region, the support struts are not protected by shielding and the hot exhaust gas will soften the struts over time, causing the struts 118 shown in FIGS. 4A-4B to deform. This deformation in turn alters the spatial location of the bearing support structure 115 and, as the location of the bearing support structure changes (i.e., sinks), the back end of the rotor lowers, which elevates the compressor end of the gas turbine. Following a small degree of such deformation and movement of the bearing support, the compressor blades impact the compressor shell, with predictable, destructive results.
To avoid such catastrophic damage, the owners of these turbines frequently incur significant outages to periodically replace damaged dead air space baffles 120. The costs of such outages include, but are not limited to, lost power generation revenue, potential contractual production penalties if the turbine generation is part of a long term power agreement, potential penalties if the turbine/HRSG steam generation is pre-sold contractually, potential loss of good-will to customers and/or business partners based on an inability to meet demand, the cost of bringing in additional plant staff and Outage Services Staff (OSS) to cover the outage (increased overhead), the cost of renting a 100-Ton crane for the duration of the outage, the cost of having scaffolding built, the cost of bringing in insulators, the cost of replacing instruments that are damaged or destroyed during disassembly, the cost of the manpower (craft laborers-millwrights/pipefitters, etc. . . . ) to perform the outage to include travel and expenses for about 8 specialized workers (for up to about 12 days), the cost of a day and night shift Craft Labor Supervisor to include travel and expenses for up to about 12 days, the cost of a day and night shift Engineer to include travel and expenses for up to about 12 days, the cost of bringing in an Alignment specialist and spending several days re-aligning the cases, the cost of a safety engineer for oversight up to about 12 days, the cost of the parts, and the cost of the startup gas. These costs can easily top $150,000 per outage.